Field of the Invention
The present invention relates to video coding. In particular, the present invention relates to coding techniques associated with the intra prediction mode coding.
Description of the Related Art
Motion compensated inter-frame coding has been widely adopted in various coding standards, such as MPEG-1/2/4 and H.261/H.263/H.264/AVC. While motion-compensated inter-frame coding can effectively reduce bitrate for compressed video, intra mode coding is still required to process an initial picture or to periodically insert I-pictures (or Intra-coded picture, I-picture), and the I-pictures are inserted periodically to allow quick access to the compressed video data or to alleviate error propagation. Intra prediction exploits the spatial correlation within a picture or within a picture region. In order to further improve coding efficiency, the High-Efficiency Video Coding (HEVC) standard being developed exploits block-based spatial prediction extensively. In HEVC, multiple intra prediction modes are used to exploit spatial features and the number of intra prediction modes depends on the block size of a Prediction Unit (PU). For some PU block sizes (currently larger than 4×4 and smaller than 64×64), the number of intra prediction modes may be as many as 35. The intra prediction mode selected for each block has to be transmitted to the decoder side for proper decoding. The side information associated with the intra prediction mode may be substantial and therefore, accordingly various predictive coding schemes to improve the efficiency for coding intra mode have been disclosed. The intra prediction modes among neighboring blocks may be highly correlated. Therefore, the intra prediction modes from neighboring blocks that have been reconstructed may be used as a predictor for a current block. In current HEVC Test Model Version 3.0 (HM-3.0), a coding scheme for the intra prediction mode is being considered, where the intra prediction mode of a current block is compared with one or two Most Probable Modes (MPMs) that are derived from the intra modes of neighboring blocks of the current block. If the intra prediction mode of the current block is the same as one most probable mode, a single-bit flag is transmitted to indicate the case. When two most probable modes are available, an index flag is transmitted to indicate which most probable mode is used. Otherwise, a single-bit flag is transmitted along with information to indicate which one of the remaining modes corresponds to the current intra mode. While the use of one or two most probable modes has helped to improve coding efficiency, the coding efficiency may be further improved by using more most probable modes.
In HM-3.0, a context-adaptive entropy coding scheme is applied to the intra coding modes. Furthermore, within the intra mode coding, Planar is mapped to mode index 2. Thus, DC and Planar modes share one mode index (i.e., 2), one codeword in context adaptive variable-length coding (CAVLC), and one binarized representation in context adaptive binary arithmetic coding (CABAC) with respect to most probable mode signaling and remaining mode coding. An additional flag is transmitted to distinguish between DC mode and Planar mode. However, the dependent coding of DC and Planar modes may degrade coding performance.